Carboxylated rubbers are polymers of a diene, a nitrile, and a monounsaturated carboxylic acid. In a preferred embodiment, carboxylic rubbers are carboxylic nitrile rubbers that are polymers of butadiene, acrylonitrile, and an acrylic acid. Carboxyl modification of nitrile rubber results in a material that has outstanding abrasion resistance. Such carboxyl modification of a rubber typically involves the addition of about 0.75 to 15 percent by weight of a monounsaturated carboxylic acid to the monomer charge being synthesized.
These carboxylated elastomers can be vulcanized in a manner analogous to their uncarboxylated counterparts utilizing a sulfur curing agent. In addition to this, if a polyvalent radical, and particularly a divalent metal, is available in the vulcanization recipe, the carboxyl groups in the polymer chain can take part in this cross-linking reaction. It is desirable to have a divalent metal in the vulcanization recipe since metal oxide vulcanizates of carboxylic rubbers or elastomers have unusually high tensile strength, superior ozone resistance, and elevated modulus values. This cross-linking reaction is fast in the presence of divalent metals and scorch problems are often encountered. Even at room temperature, carboxylated rubbers will often unintentionally cure in 48 hours or less in the presence of zinc oxide when uninhibited. Since scorch or the premature cross-linking of an elastomer can render a rubber completely unworkable, it is necessary to control this cross-linking reaction between carboxyl groups on the polymer chain.
U.S. Pat. No. 4,415,690 to Grimm discloses the use of alkyl and alkenyl succinic acids and their anhydrides in carboxylic rubber compositions to improve scorch resistance of such rubbers. Amount of the scorch inhibitor in the compositions can vary up to about 5 parts per 100 parts of rubber, on weight basis, preferably 0.1 to 1.5 phr. Although the succinic acid derivatives disclosed in the Grimm patent perform admirably as scorch inhibitors in carboxylic rubber compositions, such compositions also have cure time that is too long, and, therefore, undesirable. For instance, an uninhibited commercial carboxylic nitrile rubber composition has a cure time (t.sub.90) at 170.degree. C. of 6.7 minutes whereas another commercial nitrile rubber composition inhibited with a scorch inhibitor disclosed in the Grimm patent has a cure time (t.sub.90) at 170.degree. C. of about twice as long, or 13.5 minutes. Such a large increase in cure time is undesirable in a commercial environment since it substantially reduces productivity and concomitantly increases cost of the resulting products.